1. Technical Field
The present invention relates to an image forming apparatus that transfers a toner image on an image carrier onto a recording medium.
2. Related Art
In a typical image forming apparatus that transfers a toner image on a surface of an image carrier onto a recording medium clamped in a transfer nip, the toner image is first formed on a surface of a drum-shaped photoconductor serving as an image carrier in accordance with a well-known electrophotographic process. The photoconductor is contacted against an endless intermediate transfer belt serving as an image carrier and an intermediate transfer member, to thereby form a primary transfer nip. In the primary transfer nip, the toner image on the photoconductor is primary-transferred onto the intermediate transfer belt. The intermediate transfer belt is then contacted against a secondary transfer roller serving as a transfer member (i.e., a secondary transfer member), to thereby form a secondary transfer nip. The intermediate transfer belt is clamped between the secondary transfer roller and a secondary transfer facing roller disposed inside a loop of the intermediate transfer belt. The secondary transfer facing roller inside the loop is electrically grounded and the secondary transfer roller outside the loop is supplied with a secondary transfer bias (voltage) from a power supply, thereby forming a secondary transfer electric field for electrostatically moving the toner image from the secondary transfer facing roller side to the secondary transfer roller side in the secondary transfer nip between the secondary transfer facing roller and the secondary transfer roller. Then, with the secondary transfer electric field and a nip pressure, the toner image on the intermediate transfer belt is secondary-transferred onto a recording sheet serving as the recording medium (also referred to as a transfer sheet) transported to the secondary transfer nip in synchronization with the arrival of the toner image on the intermediate transfer belt.
If the thus-configured image forming apparatus uses a recording sheet with large surface irregularities, such as traditional Japanese paper, an uneven density pattern mirroring the surface irregularities is likely to appear in the image. The uneven density pattern is due to a failure to transfer a sufficient amount of toner to recesses in a surface of the recording sheet, making the image density lower in the recesses than in projections on the surface of the recording sheet. To address this issue, the image forming apparatus may be configured to supply not a secondary transfer bias including only a direct-current (DC) voltage but a secondary transfer bias consisting of a superimposed bias having a DC voltage superimposed with an alternating-current (AC) voltage. With such a secondary transfer bias, the appearance of the uneven density pattern is minimized more than with the secondary transfer bias including only the DC voltage.
To secondary-transfer the toner image transferred to the intermediate transfer member onto the recording sheet, the secondary transfer member such as the secondary transfer roller is normally disposed at a secondary transfer position facing a surface of the intermediate transfer member, thereby forming the secondary transfer nip between the intermediate transfer member and the secondary transfer member at the secondary transfer position. With the secondary transfer bias supplied to the secondary transfer nip, the toner image carried on the intermediate transfer member is electrostatically transferred onto the recording sheet in the secondary transfer nip.
With such a configuration, when the recording sheet is not present in the secondary transfer nip, the secondary transfer member is in contact with the surface of the intermediate transfer member. Conversely, when the recording sheet is present in the secondary transfer nip, the secondary transfer member is in contact with the rear surface of the recording sheet. Therefore, if toner on the surface of the intermediate transfer member adheres to a surface of the secondary transfer member when the recording sheet is absent in the secondary transfer nip, the toner may later adhere to the rear surface of the recording sheet, contaminating the rear surface of the recording sheet.
To prevent such contamination of the rear surface of the recording sheet, the image forming apparatus may employ a bias cleaning system to clean off the toner adhering to the secondary transfer member, without providing a cleaning device for cleaning the secondary transfer member. According to the bias cleaning system, the toner adhering to the secondary transfer member is transferred to the surface of the intermediate transfer member by a cleaning bias supplied to a secondary transfer area at a predetermined time when image formation is not taking place (e.g., before or after an image forming job), and then the toner is cleaned off by a cleaning mechanism for cleaning the intermediate transfer member.
The bias cleaning system is capable of minimizing or preventing altogether contamination of the rear surface of the recording sheet without providing the cleaning device for the secondary transfer member, and thus is advantageous in terms of a reduction in size and cost of the image forming apparatus. The bias cleaning may be performed by supplying a negative cleaning bias, which is the same as a transfer bias for transferring the toner image from the intermediate transfer member to the recording sheet, and a positive cleaning bias opposite in polarity to the negative cleaning bias. Further, the image forming apparatus may be configured to change cleaning bias supply conditions in accordance with the usage of the image forming apparatus based on the result of detection of the toner image on the intermediate transfer member to perform an appropriate amount of cleaning on the secondary transfer member.
To supply the above-described superimposed bias, however, the image forming apparatus requires a circuit for supplying the AC component. If the circuit for supplying the AC component is included in the power supply, however, a load caused by the circuit delays the rise of the DC component. Particularly if the circuit for supplying the AC component includes a capacitor, the delay in rise of the DC component is prominent. The delay of the rise time reduces the time for supplying the voltage necessary for the bias cleaning of the transfer member, which may result in insufficient cleaning and thus the contamination of the rear surface of the recording sheet in a printing operation.
To ensure sufficient cleaning performance by quickening the rise of the DC component, the image forming apparatus may be configured to output the DC component in two stages at the rise thereof. This configuration, however, increases the cost of the power supply, which increases the overall cost of the image forming apparatus. Increasing the cost of the power supply despite a low frequency of the bias cleaning of the transfer member is uneconomical. Alternatively, the overall supply time of the cleaning bias may be increased to ensure the time for the bias cleaning. In this case, there is no increase in cost of the power supply, but the bias supply time in the image transfer is also increased. Since the image transfer is frequently performed, the increase of the bias supply time in the image transfer reduces image transfer productivity.
Further, the image forming apparatus may be configured to output a large bias to improve the bias cleaning performance. This configuration, however, also causes the increase in cost of the power supply and thus the increase in overall cost of the image forming apparatus. Increasing the cost of the power supply despite the low frequency of the bias cleaning of the transfer member is uneconomical, as described above. Further, if the overall supply time of the large bias is increased, there is no increase in cost of the power supply, but the bias supply time in the image transfer process is also increased, reducing image transfer productivity owing to the high frequency of the image transfer.